As the integration density of semiconductor devices continues to increase, various research aimed at improving the productivity of semiconductor devices continues to progress. To improve the productivity of the semiconductor device, the semiconductor device should have no defects. Defects may occur at various stages of semiconductor device fabrication. Defects on the backside of a wafer may especially linger and affect subsequent processing steps.
Defects on the backside of the wafer result mainly from accumulation of unwanted particles. The particles may be dust, polymer deposits, and/or excess photo-resist accumulated during prior processing steps or wafer transfer. Such accumulation of unwanted particles may cause problems during subsequent processing steps. For example, a photoresist may cling to the backside of the wafer while a photoresist layer is forming on a front side of the wafer. The photoresist on the backside of the wafer adversely affects focusing in a subsequent lithography process and leads to a malformed pattern. This may be a major cause of defects in semiconductor devices.
In addition, in a process of forming a thin film on the front side of the wafer by chemical vapor deposition (CVD) or sputtering, backside particles prevent the wafer from sitting properly on a chuck. In such a case, the process should be suspended, which results in considerable downtime. Accordingly, backside particles deteriorate productivity and run up manufacturing costs.
An apparatus for detecting backside particles is disclosed in U.S. Pat. No. 5,963,315 entitled “Method and Apparatus for Processing a Semiconductor Wafer On a Robotic Track Having Access To In Situ Wafer Backside Particle Detection” by Hiatt, et al. According to Hiatt, et al., a laser source and a detector are mounted on a robotic arm, or within a semiconductor processing tool. While the wafer is transferred by the robotic arm, its backside is scanned by a laser beam to detect particles.
Another apparatus for detecting backside particles is disclosed in U.S. Pat. No. 6,204,917 entitled “Backside Contamination Inspection Device” by Smedt, et al. According to Smedt, et al., the semiconductor wafer is rotated to an inclined state. The wafer is supported by roller bearings and its backside is scanned by a scan head to detect particles. The scan head includes a laser illuminator and a CCD sensor and moves in close proximity to the surface being scanned to detect particles.
Generally, a laser source and a coupled sensor are used to detect backside particles. The laser source emits a laser beam onto a predetermined area of the backside of the wafer and the sensor receives a reflected beam. When particles exist on the backside of the wafer, the incident angle of the reflected beam upon the sensor varies. Backside particles can be detected by measuring the incident angle of the reflected beam. However, the laser source and the sensor should be separately mounted, thereby complicating the apparatus. Also, considerable time is required to scan the whole surface of the wafer by using the laser beam, thus delaying the overall wafer process.